The recovery of oil from subterranean oil-bearing strata is accomplished by employing several techniques that are initiated by primary recovery and often followed by various secondary and tertiary recovery procedures that are commonly referred to as enhanced oil recovery techniques. Primary oil recovery is usually achieved by penetrating the oil-bearing earth formation with a plurality of wells and recovering the oil from these wells by utilizing the natural, i.e., pore, pressure and in the subterranean earth strata. In most applicatons, especially where the oil is of relatively high viscosity or is in a "tight" formation, as little as about 5 percent of the oil is normally recovered by primary techniques.
In order to recover oil beyond the percentage attainable by primary recovery, several enhanced oil-recovering techniques have been developed and successfully utilized. For example, hydraulic fracturing, water flooding, thermal flooding, and chemical flooding have proven to be highly successful for recovering substantial oil from oil-bearing strata after completing primary recovery operations.
One of the more promising enhanced oil-recovery techniques involves the use of carbon dioxide (CO.sub.2) for flooding. In this process, carbon dioxide is pumped into a well where the earth formation is usually of a sufficiently high temperature to ensure that the carbon dioxide is in a gaseous state with the gas being readily absorbed by the oil to swell the oil and decrease the viscosity thereof. The swelling and lowering the viscosity of the oil greatly increase the mobility of the oil to facilitate its recovery from the oil-bearing strata. In some cases, the oil may be partially vaporized by the carbon dioxide allowing it to be miscibly displaced by the carbon dioxide.
Oil reservoirs throughout North America vary considerably in temperatures and pressures, with the temperatures ranging from a low of about 60.degree. F. in some of the shallower reservoirs to well over 200.degree. F. in other reservoirs. Similarly, the pore pressure varies in the range of from about several hundred psia to several thousand psia across the continent. In roughly calculating the temperature and pressure of a reservoir the standard generalized procedure is to provide an increase in temperature of about 0.02.degree. F. per foot of depth and a pore pressure increase of about 0.05 psi per fot of depth. The maximum stress or pressure that the formation at any given depth can withstand without suffering permanent damage is roughly equal to the "overburden" pressure, normally estimated at 1 psi per foot. The overburden pressure is also the maximum injection pressure that can be used in flooding operations. The fact that oil reservoirs vary considerably in temperature and pressure presents a variety of problems to the use of carbon dioxide in enhanced oil recovery procedures. Carbon dioxide is a non-polar chemical substance which has a critical temperature of 87.7.degree. F. and a critical pressure of 1,071 psia. Thus, in any oil reservoir where the reservoir temperature is less than 87.7.degree. F., the injected carbon dioxide utilized in the enhanced oil recovery will remain as a liquid when the pore pressure of the reservoir is above the cricitcal pressure. If the temperature is above the critical temperature of 87.7.degree. F., the carbon dioxide will be a gas regardless of the pore pressure. Consequently, in reservoirs where the carbon dioxide remains a liquid, the displacement and mixing of the carbon dioxide with the oil are achieved by a mixing of the liquids which at best provides only marginal enhanced recovery. Several major oil reservoirs in Canada and the United States, especially those in the Appalachian region, are at temperatures less than the critical temperature of carbon dioxide. Carbon dioxide flooding of these reservoirs would involve injection pressures greater than the saturated vapor pressure of carbon dioxide at the particular temperature of the reservoir, thus resulting in the injected carbon dioxide being in theliquid state.
A general drawback of the carbon dioxide in flooding process is that the liquid or gaseous carbon dioxide is of relatively high mobility with respect to oil, so as to have a tendency to "finger" through the oil-bearing strata towards the production well at such a rate that it has insufficient contact with the oil to provide adequate enhancement in oil recovery. Also, once the carbon dioxide reaches the production well, the enhanced oil-recovery process is essentially terminated since the carbon dioxide will tend to flow through the paths established by the "fingers" and bypass the surrounding oil-bearing strata.